For many living in a terror-spooked country, it might seem like a great government innovation: Use vans equipped with mobile X-ray units to scan vehicles at major sporting events, or even randomly, for bombs or contraband.

But news that the US is buying custom-made vans packed with something called backscatter X-ray capacity has riled privacy advocates and sparked internet worries about "feds radiating Americans."

"This really trips up the creep factor because it's one of those things that you sort of intrinsically think the government shouldn't be doing," says Vermont-based privacy expert Frederick Lane, author of "American Privacy." "But, legally, the issue is the boundary between the government's legitimate security interest and privacy expectations we enjoy in our cars."